Growth and anisotropic transport properties of off-axis MgB2 thin films

Abstract

Off-axis MgB2 thin films with c-axis tilted away from the film normal have been fabricated on MgO (211) substrate via a hybrid physical-chemical vapor deposition method. Microscopy images show that the film surface is characterized by terrace steps, which, in line with the x-ray diffraction experiment, demonstrates the tilted growth of the film. To probe the resistivity anisotropy resulted from this tilted growth, resistance measurements in Van der Pauw configuration have been performed. By refining an equivalent-circuit model proposed previously to simulate the measurement, the anisotropy ratio ρT/ρL is estimated, where ρT or ρL is the resistivity transverse or parallel to the terrace steps respectively. The validity of the estimation has been checked by direct measurements of ρT and ρL in standard linear-four probe configuration, which are performed by patterning microbridges from the film along or across the terrace steps. The patterned microbridges also enable us to measure the anisotropy of the superconducting critical current density Jc, which shows that Jc,L parallel to the terrace steps is higher than Jc,T transverse to the terrace steps and the anisotropy ratio Jc,L/Jc,T decreases as temperature increases, following a simple [1+(T/Tc)2]−1/2 dependence with Tc the superconducting transition temperature. The results are discussed under the framework of multiband transport of MgB2 which exhibits anisotropy between the crystallographic c-axis and ab-plane because of the anisotropic contribution of the σ bands.